1. Technical Field
Embodiments are generally related to air handling unit (“AHU”) and terminal boxes and particularly to air handling unit and terminal boxes used in commercial and office buildings, university and school buildings, hospital buildings, hotels, and industrial production and research facilities.
2. Discussion of Prior Art
AHU and terminal boxes are widely utilized in a variety of commercial and industrial buildings to condition and circulate air in occupied spaces and ensure occupant comfort. Typical applications of AHUs include but are not limited to single duct variable air volume air handling units, dual duct variable air volume air handling units, and multi-zone air handling units.
Terminal boxes are typically comprised of a single damper (single duct) or a plurality of dampers (dual duct), heating coil or plurality of strip electrical heaters, an airflow station, a discharge air temperature sensor, and a controller. The controller may receive information from a thermostat, an occupancy sensor, and a carbon dioxide sensor.
When a building is designed and built, an important factor that engineers must take into account is the fresh air requirement. Structures occupied by animals and humans require a specific quantity of fresh air to be habitable. Fresh air also dilutes the volatile organic compounds (“VOCs”) that may exist in any given room. ASHRAE Standard 62.2 provides a general guideline to ensure that a building meets this fresh air requirement. According to the Standard, a minimum airflow rate should be set up for each terminal box. Under design load conditions the fresh air intake is generally 10% to 20% from the air handling unit, with the minimum airflow rate often as high as 40% of the terminal box design airflow rate. Regardless of the specific building load, a constant minimum airflow rate is maintained. When the zone load served by the terminal box is lower than the minimum airflow ratio, the room temperature is maintained by reheat. Not only does keeping the airflow at a minimum constant rate fail to satisfy the fresh air requirement, but doing so results in the consumption of a significant amount of heating, cooling, and fan power energy.
The prior art includes several methods that maintain the required fresh air intake rates. In the demand based on fresh air control method, for example, a carbon dioxide sensor is installed on the return air duct to measure the concentration of carbon dioxide in the actual return air. The concentration of carbon dioxide is controlled at a rate of 700 PPM (adjustable, it is recommended by ASHRAE) higher than the concentration of carbon dioxide in the outside air. The outside air damper closes when the concentration of carbon dioxide is less than 700 PPM higher than the concentration of carbon dioxide in the outside air, and it opens more when the carbon dioxide concentration is greater than that same rate. The problem with this method, however, is twofold. First, because the occupancy rate and fresh air requirements for each zone differ, the fresh air requirement is not necessarily met in every zone. Second, the method cannot be applied in situations in which the building is lightly occupied and fresh air is predominantly used for the purpose of diluting volatile organic compounds.
A popular method in the prior art to solve the problems associated with demand based fresh air control is to set the target concentration of carbon dioxide in the return air at a lower level, for example, at 400 PPM higher than the concentration of carbon dioxide in the outside air. While this method improves the circulation of fresh air to each zone, it cannot ensure that the ventilation and fresh air requirements are satisfactorily met. Moreover, the method substantially increases the outside air intake (by as much as 40% for the entire building) as well as heating and cooling energy consumption rates. The minimum air intake ratio remains the same or is as high as 40% of the design airflow rate.
The prior art also proposes installing carbon dioxide sensors in each room to ensure that fresh air is properly distributed. This approach increases operating costs, as it requires the implementation of a significant number of carbon dioxide sensors that must be calibrated every six months. Oftentimes, these sensors give inaccurate carbon dioxide readings or are unreliable in that the expected results are not achieved. While fresh air is effectively distributed when a room is normally occupied, available fresh airflow may be reduced to zero when the room is unoccupied. In fact, during the time that the room is unoccupied, volatile organic compounds can build up and compromise the overall quality of the indoor air. Moreover, since the terminal box continues to operate at a minimum rate, excessive heating, cooling, and fan power usage may also result.
Attempting to solve the problems presented by the prior art, engineers developed a dedicated outside air intake unit that provides a constant amount of outside air to the building based on the design conditions. However, this new technology is not suitable for use under all occupancy conditions. The number of people that occupy a specific area within a building is dependent on factors that include the time of day, the day of the week, and even the particular season of the year. Therefore, due to fluctuating zone occupancy levels, the dedicated outside air intake unit often provides an excess of outside air to the building as a whole or an inadequate amount to specific, more heavily occupied zones. Further, like in the prior art, the terminal box still has a minimum airflow rate that results in excessive heating, cooling, and fan power usage.
In summary, although improvements have been made to fresh air technologies over the years, inadequate control of fresh air in a building continues to be a problem. Currently implemented methods fail to ensure that a proper amount of fresh air is distributed to each thermally controlled zone. Moreover, excessive heating, cooling, and fan power consumption results when the terminal box uses a constant minimum air flow rate. The energy wasted is often as high as 30% of the total HVAC energy consumption rate.